Systems and methods for adjusting electrical therapy based on impedance changes

ABSTRACT

System and methods for adjusting electrical therapy based on impedance changes are disclosed herein. A method in accordance with a particular embodiment includes applying a therapeutic electrical signal to a patient via an implanted portion of a patient stimulation system that includes a signal delivery device in electrical communication with a target neural population of the patient. The electrical signal is delivered in accordance with a signal delivery parameter having a first value. Using the implanted portion of the patient stimulation system, a change in an impedance of an electrical circuit that includes the signal delivery device is detected. Based at least in part on the detected impedance change, the method can further include automatically adjusting the value of the signal delivery parameter from the first value to a second value different from the first, without human intervention.

TECHNICAL FIELD

The present disclosure is directed generally to systems and methods foradjusting electrical therapy provided to patients, based on changes inthe impedance of circuits providing the therapy, including changesdetected while applying therapeutic electrical signals to a patient'sspinal cord.

BACKGROUND

Neurological stimulators have been developed to treat pain, movementdisorders, functional disorders, spasticity, cancer, cardiac disorders,and various other medical conditions. Implantable neurologicalstimulation systems generally have an implantable pulse generator andone or more leads that deliver electrical pulses to neurological tissueor muscle tissue. For example, several neurological stimulation systemsfor spinal cord stimulation (SCS) have cylindrical leads that include alead body with a circular cross-sectional shape and one or moreconductive rings spaced apart from each other at the distal end of thelead body. The conductive rings operate as individual electrodes and, inmany cases, the SCS leads are implanted percutaneously through a largeneedle inserted into the epidural space, with or without the assistanceof a stylet.

Once implanted, the pulse generator applies electrical pulses to theelectrodes, which in turn modify the function of the patient's nervoussystem, such as by altering the patient's responsiveness to sensorystimuli and/or altering the patient's motor-circuit output. In paintreatment, the pulse generator applies electrical pulses to theelectrodes, which in turn can generate sensations that mask or otherwisealter the patient's sensation of pain. For example, in many cases,patients report a tingling or paresthesia that is perceived as morepleasant and/or less uncomfortable than the underlying pain sensation.

One problem associated with existing stimulation systems is that aspectsof the systems and/or the interactions between the systems and thepatient may change over time. For example, the impedance of astimulation circuit (which includes implanted electrodes and thepatient's tissue) can change as scar tissue forms at the implant siteand/or if the lead moves within the patient, and/or if the lead becomesdisconnected or undergoes other changes. As the circuit impedancechanges, the strength of the applied signal changes, which can reducethe efficacy of the signal and/or create patient discomfort.Accordingly, there remains a need for improved techniques and systemsfor addressing patient pain in a manner that is effective andcomfortable over an extended period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partially schematic illustration of an implantable spinalcord stimulation system positioned at the spine to deliver therapeuticsignals in accordance with an embodiment of the present disclosure.

FIG. 1B is a partially schematic illustration of a lead having electrodecontacts that form elements of one or more therapy circuits havingimpedances that are detected in accordance with methods of the presentdisclosure.

FIG. 2 is a flow diagram illustrating a process for adjusting electricaltherapy parameters in response to detected changes in therapeuticcircuit impedance.

FIG. 3 is a flow diagram illustrating further details of processes fordetecting and responding to changes in therapy circuit impedance, inaccordance with further embodiments of the disclosure.

FIG. 4A is a schematic illustration of a representative detection systemin accordance with an embodiment of the disclosure.

FIG. 4B is a schematic illustration of a representative circuit fordetecting impedance changes in accordance with another embodiment of thedisclosure.

FIGS. 5A and 5B are schematic illustrations of waveforms indicatingpoints at which impedance detection may be performed in accordance withembodiments of the disclosure.

FIG. 6 is a flow diagram illustrating a process for predicting futurevalues of impedance based on impedance trends in accordance with anembodiment of the disclosure.

DETAILED DESCRIPTION

The present disclosure is directed generally to systems and methods foradjusting electrical therapy parameters based on detected changes in theimpedance of an electrical circuit that provides the electrical therapy.Specific details of certain embodiments of the disclosure are describedbelow with reference to adjusting therapy parameters for spinal cordstimulators, which may be used to treat chronic pain. However, thedisclosed systems and methods may be used in the context of otherstimulators and/or other patient conditions. Accordingly, someembodiments can have configurations, components, or procedures differentthan those described in this section, and other embodiments mayeliminate particular components or procedures. A person of ordinaryskill in the relevant art, therefore, will understand that the inventionmay have other embodiments with additional elements, and/or may haveother embodiments without several of the features shown and describedbelow with reference to FIGS. 1A-6.

Overview

During the course of a typical spinal cord stimulation therapy regimenand/or a trial period leading up to the therapy regimen, the impedanceof the electrical circuit that provides electrical therapy to thepatient may change for any variety of reasons. For example, if thepatient changes position and the implanted lead carrying the therapyelectrodes shifts, the circuit impedance may change because theelectrodes are now adjacent to tissue and/or fluid having differentimpedance characteristics. If the circuit impedance changes suddenly,the patient may experience a sudden change in the applied current and/orvoltage, potentially causing patient discomfort. In other cases, scartissue may be present at implant and/or build up on the electrodes,decreasing the efficacy with which the electrical signals are providedto the patient. In still other cases, individual electrodes may shorttogether, or the lead may become disconnected or partially disconnectedfrom the signal generator, creating one or more open circuits thatreduce or eliminate the efficacy of the therapy. In any of theseinstances, certain aspects of the following disclosure can automaticallydetect changes in circuit impedance and automatically respond to thechanges in a manner that improves patient comfort and/or improves theefficacy of the therapy.

A method in accordance with a particular embodiment of the disclosureincludes applying a therapeutic electrical signal to a patient via animplanted portion of a patient stimulation system that includes a signaldelivery device in electrical communication with a target neuralpopulation of the patient. The electrical signal is delivered inaccordance with one or more signal delivery parameters. The method canfurther include detecting a change in the impedance of an electricalcircuit that includes the signal delivery device, using the implantedportion of the patient stimulation system. Based at least in part on thedetected impedance change, the method can still further includeautomatically adjusting the value of the signal delivery parameter(s)without human intervention. Accordingly, the process of detectingimpedance changes and responding to the detected changes can beperformed autonomously by the implanted portion of the patientstimulation system.

A method in accordance with another aspect of the disclosure includesautomatically identifying changes in the impedance of an electricalcircuit that provides therapy to the patient, as a function of time.Based at least upon identifying the changes in impedance as a functionof time, the method can further include predicting a future value of theimpedance and automatically performing a task based on the predictedfuture value. Representative tasks include predicting a lead failure,confirming that an identified impedance trend conforms with an expectedtrend, and/or providing notice to a patient or practitioner of apredicted change in impedance value.

Representative Systems and Methods

In the following discussion, FIGS. 1A-1B illustrate a representativeimplementation of a system implanted in the patient's spinal cordregion, and FIGS. 2-6 illustrate representative methods and associatedcircuits and waveforms for detecting impedance changes and responding tothe detected changes. FIG. 1A schematically illustrates a representativetreatment system 100 for providing relief from chronic pain and/or otherconditions, arranged relative to the general anatomy of a patient'sspinal cord 191. The system 100 can include a pulse generator 101, whichmay be implanted subcutaneously within a patient 190 and coupled to asignal delivery element 109. In a representative example, the signaldelivery element 109 includes a lead or lead body 110 that carriesfeatures or elements for delivering therapy to the patient 190 afterimplantation. The pulse generator 101 can be connected directly to thelead body 110, or it can be coupled to the lead body 110 via acommunication link 102 (e.g., an extension). As used herein, the termslead and lead body include any of a number of suitable substrates and/orsupport members that carry devices for providing therapy signals to thepatient 190. For example, the lead body 110 can include one or moreelectrodes or electrical contacts that direct electrical signals intothe patient's tissue, such as to provide for patient relief. In otherembodiments, the signal delivery element 109 can include devices otherthan a lead body (e.g., a paddle) that also direct electrical signalsand/or other types of signals to the patient 190.

The pulse generator 101 can transmit signals to the signal deliveryelement 109 that up-regulate (e.g., stimulate or excite) and/ordown-regulate (e.g., block or suppress) target nerves. As used herein,and unless otherwise noted, the terms “stimulate” and “stimulation”refer generally to signals that have either type of effect on the targetnerves. The pulse generator 101 can include a machine-readable (e.g.,computer-readable) medium containing instructions for generating andtransmitting suitable therapy signals. The pulse generator 101 and/orother elements of the system 100 can include one or more processors 107,memories 108 and/or input/output devices. Accordingly, the process ofdetermining impedance and taking follow-up action can be performed bycomputer-executable instructions contained on computer-readable media,e.g., the processor(s) 107 and/or memory(s) 108. The pulse generator 101can include multiple portions, elements, and/or subsystems (e.g., fordirecting signals in accordance with multiple signal deliveryparameters), housed in a single housing, as shown in FIG. 1A, or inmultiple housings. In any of these embodiments, the pulse generator 101and/or other implanted components of the system 100 can include elementsfor detecting and responding to impedance changes.

In some embodiments, the pulse generator 101 can obtain power togenerate the therapy signals from an external power source 103. Theexternal power source 103 can transmit power to the implanted pulsegenerator 101 using electromagnetic induction (e.g., RF signals). Forexample, the external power source 103 can include an external coil 104that communicates with a corresponding internal coil (not shown) withinthe implantable pulse generator 101. The external power source 103 canbe portable for ease of use.

In another embodiment, the pulse generator 101 can obtain the power togenerate therapy signals from an internal power source, in addition toor in lieu of the external power source 103. For example, the implantedpulse generator 101 can include a non-rechargeable battery or arechargeable battery to provide such power. When the internal powersource includes a rechargeable battery, the external power source 103can be used to recharge the battery. The external power source 103 canin turn be recharged from a suitable power source (e.g., conventionalwall power).

In some cases, an external programmer 105 (e.g., a trial stimulator) canbe coupled to the signal delivery element 109 during an initial implantprocedure, prior to implanting the pulse generator 101. For example, apractitioner (e.g., a physician and/or a company representative) can usethe external programmer 105 to vary the stimulation parameters providedto the signal delivery element 109 in real time, and select optimal orparticularly efficacious parameters. During this process, thepractitioner can also vary the position of the signal delivery element109. After the position of the signal delivery element 109 andappropriate signal delivery parameters are established using theexternal programmer 105, the patient 190 can receive therapy via signalsgenerated by the external programmer 105, generally for a limited periodof time. In a representative application, the patient 190 receives suchtherapy for one week. Assuming the trial therapy is effective or showsthe promise of being effective, the practitioner then replaces theexternal programmer 105 with the implanted pulse generator 101, andprograms the pulse generator 101 with parameters selected based on theexperience gained during the trial period. Optionally, the practitionercan also replace the signal delivery element 109. Once the implantablepulse generator 101 has been positioned within the patient 190, thesignal delivery parameters provided by the pulse generator 101 can stillbe updated remotely via a wireless physician's programmer (e.g., aphysician's remote) 111 and/or a wireless patient programmer 106 (e.g.,a patient remote). Generally, the patient 190 has control over fewerparameters than does the practitioner. For example, the capability ofthe patient programmer 106 may be limited to starting and/or stoppingthe pulse generator 101, and/or adjusting stimulation amplitude.

In any of the foregoing embodiments, the parameters in accordance withwhich the pulse generator 101 provides signals can be modulated duringportions of the therapy regimen. For example, the frequency, amplitude,pulse width, duty cycle, and/or signal delivery location can bemodulated in accordance with a preset program, patient and/or physicianinputs, and/or in a random or pseudorandom manner. Such parametervariations can be used to address a number of potential clinicalsituations, including changes in the patient's perception of pain,changes in the preferred target neural population, and/or patientaccommodation or habituation. Such parameter changes can also beimplemented automatically to account for detected impedance changes.

FIG. 1B illustrates a representative lead 110 that can be connected tothe pulse generator 101. The lead 110 can have any suitable number ofcontacts C positioned along its length L for delivering electricaltherapy to the patient. For purposes of illustration, 11 contacts C,identified individually as contacts C1, C2 . . . C11 are shown in FIG.1B. In operation, one or more of the contacts C is cathodic and anotherone or more of the contacts C is anodic. The contacts C can beindividually addressable so that any contact C or combination ofcontacts C can operate as a cathode, and any contact C or combination ofcontacts C can operate as an anode. The contacts C can be electricallygrouped in any of a wide variety of combinations, and individualcontacts C can perform different functions (e.g., cathodic functionsand/or anodic functions) at different times during the course of atherapy regimen. In any of these embodiments, each contact C may becoupled with a corresponding conductor 111 to the pulse generator 101.The conductors 111 may have one or more connection points along theirlengths (e.g., at a junction with the pulse generator 101, andoptionally at a junction with an extension). Accordingly, the circuitfor a given pair of contacts C includes the contacts C, the patienttissue T between the contacts, the individual conductors 111, connectionpoints along the conductors 111, and connection points between theconductors 111 and the pulse generator 101. As discussed below, aspectsof the present disclosure are directed to detecting changes in theoverall circuit impedance that may result from changes at any of avariety of points along the circuit, and providing an appropriateresponse.

FIG. 2 illustrates an overall process 220 in accordance with aparticular embodiment of the disclosure. The process 220 includesapplying electrical therapy to a patient via an implanted or partiallyimplanted system (process portion 225). During the course of thetherapy, the process 220 can further include detecting a change in theimpedance of an electrical circuit that provides the therapy, using theimplanted portion of the system (process portion 226). Based on thedetected impedance change, the process 220 can still further includeautomatically adjusting signal delivery parameters in accordance withwhich the therapy is provided, without human intervention (processportion 227). The signal delivery parameters can include signalstrength, the location and/or number of electrical contacts deliveringtherapy to the patient, and/or other parameters. FIG. 3 describesfurther aspects of particular embodiments of the foregoing process.

FIG. 3 is a block diagram illustrating in further detail aspects of theoverall process 220 described above with reference to FIG. 2. Processportion 221 includes identifying a target neural population. Forexample, process portion 221 can include identifying one or more neuralpopulations at particular vertebral locations along the patient's spine.Process portion 222 includes implanting an electrical therapy system,for example, implanting the signal delivery element 109 and the pulsegenerator 101 described above with reference to FIGS. 1A and 1B. Asdiscussed above with reference to FIG. 1B, the electrical therapy systemincludes at least one electrical therapy circuit which has associatedwith it an initial impedance value. In process portion 225, electricaltherapy is applied to the patient via the implanted system with theinitial impedance value. The impedance value may change over time, whichis detected in process portion 226. The electrical therapy signals aretypically time-varying signals (e.g., pulses), and the associatedcircuit can include capacitive and/or resistive elements. Accordingly,the detected impedance can include capacitive and/or resistivecomponents. In particular embodiments, the system can distinguishbetween resistive and capacitive components of the overall impedance,and use this information to select the appropriate signal deliveryparameter to vary. For example, if the impedance change is primarilycapacitive, changing the signal delivery frequency may have a greatereffect than if the impedance change is primarily resistive.

The change in impedance of the electrical circuit can be detected bydetecting a change in power applied to the circuit (process portion227), a change in voltage at one or more points along the circuit(process portion 228), a change in current passing through the circuit(process portion 229), and/or changes in other values. In a particularembodiment (process portion 230), the impedance of the electricaltherapy circuit is detected while the therapy signal is delivered, forexample, by sampling the therapy signal itself. In other embodiments, adedicated electrical signal can be applied to the electrical therapycircuit and sampled, so as to detect impedance and/or impedance changes,independent of the therapy signal (process portion 231). Accordingly,this arrangement can be used to detect impedance and/or impedancechanges of a therapy circuit without the need for the therapy signal tobe active on the circuit at that time.

In any of the foregoing embodiments, the process 220 can includedifferent steps depending upon the magnitude and/or direction of theimpedance change. In a particular embodiment, process portion 232 caninclude determining whether the impedance change is an increase or adecrease. If the change is an increase, process portion 233 can includedetermining whether the circuit is open. An open circuit can beindicated by an infinite impedance, or by an impedance or impedanceincrease that exceeds a suitable threshold value.

Process portions 234-240 may be performed if, in process portion 233, itis determined that the electrical therapy circuit is open. For example,process portion 234 can include reducing the signal strength (e.g., theapplied current and/or voltage) prior to closing or attempting to closethe open circuit (process portion 235). In particular embodiments,process portion 234 can include reducing the signal strength to zero, orto a suitable non-zero value such that when the circuit is later closed,the patient does not receive an uncomfortable sensation. Process portion235 can, for example, include changing one or more of the contacts(e.g., those shown in FIG. 1B) that form a portion of the electricaltherapy circuit. This change can be achieved by opening and closingappropriate switches at the pulse generator 101. Once the change hasbeen made, process portion 236 can include gradually increasing thestrength of the signal applied to the therapy circuit. By first reducingthe signal strength (process portion 234) and then gradually increasingthe signal strength after the change in the circuit has been made(process portion 236), the process 220 can avoid subjecting the patientto a sudden increase in the applied signal, which may cause patientdiscomfort.

Once the signal strength has been increased, process portion 237 caninclude checking the impedance to determine whether it has returned toits original level, or has otherwise achieved an acceptable level. Thisprocess can include checking the power, voltage, and/or current, asdescribed above with reference to process portions 227-229. If theimpedance is at an acceptable level, process portion 238 includescontinuing to deliver the therapy. In addition, the patient and/orpractitioner can be notified that a change has been made to account forthe detected increase in impedance. The notification can take any of avariety of suitable forms, including a vibratory signal, and/or aninternally stored signal that is downloaded external to the patientduring a battery recharge operation or diagnostic operation. If thechange did not result in an adequately improved impedance level, then inprocess portion 239, the system checks to determine whether all thecontacts of the implanted lead have been checked. If not, the processreturns to process portion 234 and a different contact is placed in thecircuit. If, after all of the contacts have been checked and determinedto form an open circuit, this can indicate that the lead isdisconnected, which is flagged for the user and/or practitioner inprocess portion 240, using any suitable notification technique. Inprocess portion 250, the system checks for lead reconnection, e.g., byrechecking all contacts for an open circuit. Once the lead has beenreconnected, the system returns to process portion 236 so as togradually increase signal strength.

If in process portions 232 and 233, the system determines that theimpedance has decreased, or has increased but not so much as to indicatean open circuit, then in process portion 241, the signal deliveryparameters are adjusted in an autonomous manner to bring the impedancevalues to acceptable levels. Process portion 241 can in turn includechanging any of a variety of signal delivery parameters as identified byprocess portions 242-245. For example, process portion 242 can includechanging the number of active contacts in the electrical therapycircuit. The number of active contacts may be increased in particularembodiments if the circuit impedance increases (e.g., to provideparallel electrical paths), and may be decreased in other embodiments ifthe impedance decreases. In a particular example, the patient may changeposition or the lead may change position relative to the patient suchthat the circuit impedance increases because the active contacts are nowpositioned adjacent to a portion of the patient tissue having a higherimpedance. By increasing the number of active contacts, the resultingcircuit can include portions of the patient tissue having a lowerimpedance. Of course, when such a change is made, it is made so that theresulting therapy is still applied to the appropriate portion of thepatient tissue, even though the range over which the active contacts arepositioned and may now be expanded. For example, only a subset of thecontacts shown in FIG. 1B may be used and/or available for such achange, so as to avoid directing the therapy signal too far from theoriginally intended neural population.

In other embodiments, the system can automatically detect a decrease inthe impedance of the therapy circuit. For example, if the lead shifts toa location within the patient that results in a lowered impedance, thesystem can automatically detect this occurrence. One such example is ifthe lead shifts from an initial position so as to be in closer proximityto cerebral spinal fluid, which has a relatively low impedance. In suchinstances, the number of electrical contacts applying the therapy may bedecreased. In any of the foregoing embodiments once the responsivechange has been made, the process can continue with process portion 251,which includes determining whether the new circuit produces a targetimpedance value. If so, then in process portion 238, the system deliversadditional therapy. In addition, the system can notify the patientand/or practitioner of the update. If the change has not produced atarget impedance value, the process returns to process portion 241. Atthat point, process portion 242 can be re-executed using a differentcombination of electrodes, or any of process portions 243-245 can beexecuted.

Process portion 243 includes changing the contacts that are active, forexample, without increasing or decreasing the number of active contacts.In a particular example, process portion 243 can include substitutingone contact for another. The substituted contact can be an anodiccontact or a cathodic contact, and in some embodiments, both anodic andcathodic contacts can be changed. Once the contacts have been changed,the impedance is checked (process portion 251) and the process continuesin a manner generally similar to that discussed above.

Process portion 244 includes adjusting the signal strength (increasingor decreasing the strength) in response to an indication that animpedance has changed. The signal strength can correspond to a voltagelevel, a current level, and/or a power level. Process portion 244 can inturn include selecting an updated strength value (e.g., an updatedvoltage value or current value) as shown in process portion 252, andthen determining whether the updated strength value exceeds apredetermined compliance value (process portion 246). For example, atypical constant current source provides signals at a constant currentby modulating the applied voltage, but keeping the applied voltage belowa predetermined compliance voltage value, e.g., a programmed compliancevoltage. If the selected updated strength value does not result in thecompliance value being exceeded, then the impedance can optionally berechecked in process portion 251 and the system can then deliveradditional therapy (process portion 238). The impedance check at processportion 251 may be optional in this case because changing the signalstrength may not change impedance.

If, in process portion 246 it is determined that the compliance value isexceeded by the selected updated strength value, then in process portion247, the compliance value is adjusted. As discussed above, a constantcurrent source automatically adjusts the applied voltage to account forat least some changes in the circuit impedance, so as to effectivelyproduce a constant current. If the circuit impedance increases beyond alevel that can be accommodated by a voltage change that remains withinthe maximum compliance voltage value, the compliance voltage can beadjusted, within preset limits that are established to provide forpatient safety and comfort. Accordingly, in process portion 248, thesystem determines whether such a preset limit on the degree to which thecompliance voltage can be adjusted has been exceeded. If it has not,then the impedance can optionally be checked (process portion 251) andthe therapy delivered to the patient (process portion 238). If thenecessary compliance value exceeds the preset limit, then in processportion 249, the user and/or the practitioner can be notified that therecommended signal strength is not available without exceeding presetlimits on the amount by which the compliance value can be changed. Inaddition to or in lieu of providing such notification, the process canautomatically revert to one of the other available methods for adjustingthe signal delivery parameters, including process portion 242 (changingthe number of active contacts), process portion 243 (changing whichcontacts are active), and/or process portion 245 (changing otherparameters). The other parameters that may be changed can include, butare not limited to, frequency, pulse width, and/or duty cycle.

In some embodiments, the compliance voltage value is increased, e.g., toaccount for an increased circuit impedance. In other embodiments, thecompliance voltage value is decreased, e.g., in response to an impedancereduction. In any of the foregoing embodiments, it can be advantageousto keep the compliance value as low as possible, while still providingsignals at an effective signal strength, so as to reduce the powerconsumed by the system. Accordingly, the system can automatically adjustthe compliance margin (e.g., the difference between the compliancevoltage and actual delivered voltage). For example, the system canautomatically set the compliance margin to be a fixed level (in volts ormillivolts) above the actual delivered voltage (e.g., the updatedstrength value). In other embodiments, the compliance margin can be afixed percentage of the updated strength value. In still furtherembodiments, the compliance margin can vary, and can be automaticallyadjusted based on the range of requested strength perturbations, e.g.,the degree to which the updated strength values deviate from thestrength value being applied at that time. If the range is wide, thecompliance margin can be correspondingly wide to avoid having torepeatedly adjust the compliance value. If the range is narrow, thecompliance margin can be correspondingly narrow to reduce powerconsumption. Accordingly, in particular embodiments, the system canautomatically track the foregoing perturbations and automatically adjustthe compliance margin.

In particular instances, the foregoing arrangement can track the gradualreduction in the range of impedance perturbations that result when anewly implanted lead becomes “scarred in.” The system can also trackincreases in the range of impedance perturbations if the scar tissuecontinues to build up in a manner that requires frequent upwardadjustments in signal strength, and/or if the lead and/or patientundergo other changes that affect circuit impedance.

The systems and methods described above can operate in accordance with avariety of different modes. For example, during a first mode ofoperation, the system can check the impedance of one or more circuitsthat include only active therapy contacts. If, during the first mode ofoperation, the system identifies an impedance change in association withthe active contact(s), it can then check the impedance of inactivecontacts during a second mode of operation. Based on the impedancemeasurements obtained during the second mode, the system canautomatically change which contacts are active and which are inactive.This arrangement can reduce the time and power required to perform animpedance check because inactive contacts are checked only when animpedance change is identified in association with one or more activecontacts.

One feature of several embodiments described herein is that the system100 can automatically identify changes in impedance via an implantedportion of the system, and automatically take appropriate action withoutpatient or practitioner intervention. An advantage of this feature isthat the system need not rely on external analysis and/or diagnostics,and accordingly can implement the action more quickly and/or moreconveniently.

Another feature of several of the embodiments described herein is thatthe system 100 can automatically identify an open circuit (e.g., adisconnected lead) and then gradually increase the strength of theapplied therapy signal once the open circuit has been closed. Anadvantage of this feature is that it can reduce or eliminate thelikelihood for the patient to feel a sudden shock or discomfort, whichmay otherwise result when the therapy signal is suddenly re-applied atthe target strength value.

Still another feature of several of the embodiments described herein isthat the system 100 can automatically adjust a preset compliance level(e.g., a voltage or current) to account for impedance changes. Whileincreasing the compliance level may reduce overall system efficiency(e.g., may result in an increased power consumption), the patient canreceive dynamically updated and effective therapy in a short period oftime. This arrangement can allow the patient to continue receivingeffective therapy until the power source is recharged or replaced, asopposed to potentially requiring that the patient undergo an in-officeprocedure to address the source of the impedance change (e.g., leadmovement or scar tissue).

Yet another feature of several of the disclosed embodiments is that thesystem can detect impedance changes while the patient is receivingtherapy. Accordingly, the patient's therapy need not be interrupted toprovide this function. In addition, the system can not only check theimpedance of the lead contacts applying therapy at a given time, butalso other contacts. Accordingly, the system can readily identifyalternate contacts that may be substituted for or provided in additionto the currently active contacts, to address changes in impedance. Inaddition, this arrangement allows the system to rapidly identifyconditions that may affect contacts other than just the currently activecontacts. Such conditions include a disconnected lead.

FIG. 4A is a schematic illustration of portions of the system 100 usedfor impedance detection in accordance with an embodiment of thedisclosure. The system 100 can include an impedance detector 119 that inturn includes a microcontroller or microprocessor 107 that is coupled toan analog-to-digital (A/D) converter 112. The ND converter 112 iscoupled to a stimulation circuit 113 that includes one or moreelectrical contacts, and is implanted in the patient 190. Accordingly,the A/D converter 112 converts analog signals (e.g., voltage and/orcurrent levels) to digital values, which are analyzed by themicroprocessor 107 to identify impedance changes.

FIG. 4B is a representative circuit diagram of an embodiment of thesystem 100 described above with reference to FIGS. 1 and 4A. In thisparticular embodiment, the A/D converter 112 is connected to a voltagedivider 114, which is in turn selectively couplable to the stimulationcircuit 113 via an enable measurement switch 115. In other embodiments,the voltage divider 114 can be replaced with other suitable elements,for example, an amplifier. In any of these embodiments, the stimulationcircuit 113 can include a constant current source 116 that receives acompliance voltage input 117, which can in turn be initially preset andthen optionally adjusted within further preset limits, as discussedabove. The constant current source 116 can be coupled to a contactselector 118, which is in turn connected to the contacts C1, C2 . . .C11 positioned at the patient 190. The contact selector 118 can includeany of a wide variety of suitable switching devices used to select oneor more of the contacts C1-C11 for applying therapy to the patient,and/or for impedance detection. The circuit can include an H-bridge orother suitable element to selectively reverse the current direction forone or more combinations of contacts C. In other embodiments, the system100 can include other suitable arrangements, for example, a constantvoltage source rather than a constant current source, a current sink inaddition to the constant current source, and/or other suitablearrangements known to those of ordinary skill in the relevant art.

In any of the foregoing embodiments, the detector 119 will typicallydetect the impedance of a circuit that includes at least two contact,e.g., C1 and C2. If the circuit impedance increases beyond a presetthreshold, the system can automatically step through the remainingcontacts, e.g., C1 in combination with C3, C1 in combination with C4,and so on. This sequential process can be completed for any combinationof contacts that may be suitable for continuing the patient therapy. Inany of these cases, however, the impedance of the tested circuit willinclude the impedances of all the contacts in the circuit. Accordingly,the microprocessor 107 can include one or more of a variety ofprovisions for isolating impedance changes to a particular contact orcontacts. For example, in one case, a nominal impedance can be assignedto other contacts included in the circuit (e.g., if multiple contactsare coupled in parallel in the circuit), and then subtracted from atotal impedance to obtain the impedance for a given contact. In anotherembodiment, a series of multivariable equations can be used to solve forthe impedance of each individual contact. Such an implementation isexpected to consume more power than others, and accordingly, may in atleast some embodiments be selected only when suitable power isavailable. In another approach, the microprocessor 107 can employ asuccessive approximation technique by using an initially measured valuefor one contact to estimate values for other contacts, and thenrepeating the process as the impedances of circuits with other contactsare identified. In any of the foregoing embodiments, the detector 119obtains impedance values and/or changes in impedance values which areprovided as inputs to other functional elements of the microprocessor107 that automatically implement the changes described above withreference to FIGS. 2 and 3.

FIG. 5A illustrates a representative waveform 570 a having multiplepulses 571, each with a cathodic phase 572 and an anodic phase 573 fordelivering therapy to the patient. In a particular aspect of thisembodiment, the waveform 570 a can be sampled directly (e.g., at thecathodic phase 572) via the circuit described above with reference toFIG. 4B, to identify circuit impedance. For example, arrow S identifiesa suitable sampling point. In particular embodiments, the waveform 570 acan be sampled at multiple points, and the results analyzed to provide acomplex impedance value.

FIG. 5B illustrates another waveform 570 b having multiple bursts ofpulses 571 separated by a quiescent period. In this embodiment, aseparate detection pulse 574 can be superimposed on the waveform 570 bat the quiescent period. The impedance characteristics of the circuitare accordingly determined by analyzing the impedance associated withthe detection pulse 574. In still further embodiments, the detectionpulse 574 can be applied to a circuit that is not applying therapy atthat time, in which case the waveform 570 b is entirely quiescent exceptfor the presence of the detection pulse 574. In any of theseembodiments, the detection pulse 574 is typically provided atsubthreshold leads to avoid triggering a motor and/or sensory responsein the patient.

In any of the foregoing embodiments, the therapy signal can be appliedat frequencies ranging from about 1.5 kHz to about 100 kHz, duty cyclesof from about 50% to about 100%, e.g., with the stimulation signal onfor a period of about 10 milliseconds to about 2 milliseconds, and offfor a period of about 1 millisecond to about 1.5 milliseconds. Theapplied pulses can have a width of from about 30 to 35 microseconds andan amplitude in the range of about 1 mA to about 4 mA. Otherrepresentative parameters are included in co-pending U.S. ProvisionalApplication No. 61/176,868, incorporated herein by reference. In otherembodiments, these parameters can have different values suitable forproviding patient therapy to any of a variety of neural populations toaddress any of a variety of patient conditions.

FIG. 6 is a flow diagram illustrating a process 620 in accordance withanother embodiment of the disclosure. Process 620 can include applyingelectrical therapy to the patient (process portion 625), andautomatically identifying changes in the impedance of an electricalcircuit via which the therapy is applied, as a function of time (processportion 626). Based on the information obtained in process portion 626,process portion 627 can include predicting a future value of theimpedance. For example, the future value can be predicted by a suitableextrapolation technique, including a linear or higher order function, aleast-squares curve fit, or other technique. Based on the predictedfuture value of the impedance, process portion 628 can includeautomatically performing a task, e.g., without human intervention.Representative tasks can include predicting a lead failure, e.g., if thelead impedance intermittently becomes excessive. In other embodiments,process portion 628 can include predicting excessive impedance such asmay result from the formation of scar tissue around a lead, for example,if the impedance exhibits a steady growth. Still another function thatmay be performed via process portion 628 includes confirming that theimpedance trend identified in process portion 626 conforms with anexpected trend, for example, an expected trend or schedule resultingfrom an expected amount of scar tissue growth. In still anotherembodiment, the task can include updating any of the signal deliveryparameters described above, and/or adjusting the compliance valuesand/or compliance margins, as discussed above. In any of theseembodiments, process portion 628 can include notifying the patientand/or practitioner, alone or in combination with any of the foregoingactivities.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosure have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, while certain aspects of the disclosure weredescribed above in the context of spinal cord stimulation to treatchronic pain, similar techniques may be used in other embodiments toidentify impedance changes and/or trends for therapy applied to otherneural populations and/or for other patient conditions. The signaldelivery parameters described above may have other values in otherembodiments, for example, lower frequencies than the 1.5 kHz-100 kHzrange described above. The detection circuits can have arrangementsother than those specifically disclosed herein. The microprocessor canimplement responses to the detected impedance changes using any of avariety of suitable programming languages and techniques.

Certain aspects of the foregoing disclosure described in the context ofparticular embodiments may be combined or eliminated in otherembodiments. For example, the process of predicting impedance trendsdiscussed above with reference to FIG. 6 may be combined with theprocess for adjusting impedance values described above with reference toFIGS. 2 and 3. The impedance values may be tracked in a manner that iscoordinated with patient position or activity, as is disclosed inco-pending U.S. Provisional Application No. 61/151,464, incorporatedherein by reference. The method of tracking impedance changes as afunction of time and predicting future impedance values need not beperformed by an implanted portion of the system in some embodiments, andcan instead be performed outside the patient. Further, while advantagesassociated with certain embodiments have been described in the contextof those embodiments, other embodiments may also exhibit suchadvantages, and not all embodiments need necessarily exhibit suchadvantages to fall within the scope of the present disclosure.

1. A method for controlling a patient therapy device, comprising:applying a therapeutic electrical signal to a patient via an implantedportion of a patient stimulation system that includes a signal deliverydevice in electrical communication with a target neural population ofthe patient, the electrical signal being delivered in accordance with asignal delivery parameter having a first value; using the implantedportion of the patient stimulation system, detecting a change in animpedance of an electrical circuit that includes the signal deliverydevice; and based at least in part on the detected impedance change,automatically adjusting the value of the signal delivery parameter fromthe first value to a second value different than the first value,without human intervention.
 2. The method of claim 1 whereinautomatically adjusting the value of the signal delivery parameterincludes halting delivery of the electrical signal.
 3. The method ofclaim 1 wherein automatically adjusting the value of the signal deliveryparameter includes changing at least one of a strength, pulse width, andduty cycle of the signal.
 4. The method of claim 1 wherein theelectrical signal is applied to a first contact set, and whereinautomatically adjusting the value of the signal delivery parameterincludes applying the electrical signal to a second contact set having adifferent member than the first contact set.
 5. The method of claim 1wherein detecting a change in an impedance of an electrical circuitincludes detecting a change in an impedance of an electrical circuitthat includes the target neural population.
 6. The method of claim 1wherein applying a therapeutic electrical signal includes applying theelectrical signal via an implanted electrical contact carried by thesignal delivery device and exposed to the patient's tissue.
 7. Themethod of claim 1 wherein applying a therapeutic electrical signalincludes applying the signal to the patient's spinal cord region.
 8. Themethod of claim 1 wherein detecting a change in an impedance includesdetecting an increase in impedance.
 9. The method of claim 8 whereinautomatically adjusting the value of the signal delivery parameterincludes increasing a number of implanted electrical contacts to whichthe electrical signal is directed.
 10. The method of claim 8 wherein theelectrical signal is applied to a first implanted electrical contactset, and wherein automatically adjusting the value of the signaldelivery parameter includes applying the electrical signal to a secondimplanted electrical contact set having different members than the firstcontact set.
 11. The method of claim 8 wherein automatically adjustingthe value of the signal delivery parameter includes increasing astrength of the signal.
 12. The method of claim 8 wherein the implantedportion of the stimulation system includes a constant current sourcehaving a compliance voltage level with a first value, and whereinautomatically adjusting the value of the signal delivery parameterincludes: increasing the compliance voltage level from the first valueto a second value higher than the first; and increasing a strength ofthe signal beyond a strength available with the first value.
 13. Themethod of claim 8 wherein detecting an increase in an impedance includesdetecting an increase correlated with an increase in power provided tothe signal delivery device.
 14. The method of claim 8 wherein detectingan increase in an impedance includes detecting an open circuit.
 15. Themethod of claim 14 wherein automatically adjusting the value of thesignal delivery parameter includes reducing a strength of the electricalsignal and then ramping up the strength of the electrical signal afterthe circuit is closed.
 16. The method of claim 1 wherein detecting achange in an impedance includes detecting a decrease in impedance. 17.The method of claim 16 wherein automatically adjusting the value of thesignal delivery parameter includes decreasing a strength of theelectrical signal.
 18. The method of claim 16 wherein detecting adecrease includes detecting a decrease caused by proximity of theelectrical contacts of the signal delivery device to cerebral spinalfluid.
 19. The method of claim 16 wherein detecting a decrease includesdetecting a decrease caused by an electrical short between electricalcontacts of the signal delivery device.
 20. The method of claim 1wherein detecting a change in an impedance includes detecting a changein impedance resulting from a build up of scar tissue on the signaldelivery device.
 21. The method of claim 1 wherein detecting a change inan impedance includes detecting a change in impedance resulting from achange in position of the signal delivery device relative to thepatient.
 22. The method of claim 1 wherein detecting a change in animpedance includes detecting a change in impedance resulting from changein the patient's posture.
 23. The method of claim 1 wherein detecting achange in an impedance includes detecting a change in impedance as afunction of time and wherein the method further comprises establishing atrend for future changes in impedance.
 24. The method of claim 1 whereindetecting a change in an impedance includes detecting a change inimpedance by monitoring the therapeutic electrical signal.
 25. Themethod of claim 1 wherein detecting a change in an impedance includesapplying an electrical signal in addition to the therapeutic electricalsignal and detecting a change in impedance by monitoring the additionalelectrical signal.
 26. The method of claim 1 wherein automaticallyadjusting a signal delivery parameter includes automatically adjusting acompliance value in accordance with which the electrical signal isdelivered, in response to the detected impedance change.
 27. The methodof claim 1 wherein automatically adjusting a signal delivery parameterincludes automatically adjusting a compliance margin in accordance withwhich the electrical signal is delivered, in response to the detectedimpedance change.
 28. A method for controlling a patient therapy device,comprising: applying a therapeutic electrical signal to a patient via apatient stimulation system that includes a signal delivery electricalcontact in electrical communication with a target neural population ofthe patient, the electrical signal having a signal delivery parameterwith a first value; automatically identifying changes in an impedance ofan electrical circuit that includes the signal delivery electricalcontact, as a function of time; based at least in part on identifyingthe changes in impedance as a function of time, predicting a futurevalue of the impedance; and based at least in part on the predictedfuture value of the impedance, automatically performing a task.
 29. Themethod of claim 28 wherein automatically performing a task includesautomatically comparing the identified changes in impedance as afunction of time with a target rate of impedance change.
 30. The methodof claim 28 wherein automatically performing a task includesautomatically adjusting the value of the signal delivery parameter fromthe first value to a second value different than the first value,without human intervention.
 31. The method of claim 28 whereinautomatically performing a task includes: automatically adjusting thevalue of the signal delivery parameter from the first value to a secondvalue different than the first value, without human intervention; andapplying a second therapeutic electrical signal to the patient using thesecond value of the signal delivery parameter.
 32. The method of claim28 wherein automatically identifying changes in an impedance as afunction of time includes automatically identifying the changes as afunction of time using a portion of the patient stimulation system thatis implanted in the patient, without human intervention.
 33. The methodof claim 28 wherein automatically identifying changes in an impedance asa function of time includes automatically identifying the changes as afunction of time using a portion of the patient stimulation system thatis external to the patient.
 34. The method of claim 28 whereinpredicting a future value of the impedance includes extrapolating thefuture value based on past values.
 35. A patient therapy system,comprising: a patient-implantable portion that includes a pulsegenerator and a signal delivery device having an electrical contactpositioned to be in electrical communication with a target neuralpopulation of a patient when implanted; an impedance detector carried bythe implantable portion, the impedance detector being operativelycoupled to an electrical circuit that includes the electrode to detectan impedance of the electrical circuit; and a processor carried by thepatient-implantable portion, the processor having a computer-readablemedium programmed with instructions that, when executed: receive anindication of an impedance change from the impedance detector; and basedat least in part on the detected impedance change and without humanintervention, automatically adjust a value of a signal deliveryparameter in accordance with which the patient-implantable portiondelivers an electrical signal to the electrical contact.
 36. The systemof claim 35 wherein the processor is programmed with instructions that,when executed, automatically adjust an amplitude of the electricalsignal.
 37. The system of claim 35 wherein the processor is programmedwith instructions that, when executed, automatically terminate theelectrical signal.
 38. The system of claim 35 wherein the electricalsignal is applied to a first contact set, and wherein automaticallyadjusting the value of the signal delivery parameter includes applyingthe electrical signal to a second contact set having a different memberthan the first contact set.
 39. The system of claim 35 wherein theimplantable portion includes a constant current source having acompliance voltage level with a first value, and wherein automaticallyadjusting the value of the signal delivery parameter includes:increasing the compliance voltage level from the first value to a secondvalue higher than the first; and increasing a strength of the signalbeyond a strength available with the first value.
 40. The system ofclaim 35 wherein the implantable portion includes a constant currentsource having a compliance voltage level with a first value, and whereinautomatically adjusting the value of the signal delivery parameterincludes decreasing the compliance level from the first value to asecond value less than the first value.
 41. The system of claim 35wherein receiving an indication of an impedance change includesreceiving an indication of an open circuit.
 42. The system of claim 41wherein automatically adjusting the value of the signal deliveryparameter includes reducing a strength of the electrical signal and thenramping up the strength of the electrical signal after the circuit isclosed.
 43. The system of claim 35 wherein the pulse generator outputs atherapeutic electrical signal and a diagnostic signal, and wherein theimpedance detector is coupled to the electrical circuit to detect acharacteristic of a diagnostic electrical signal.
 44. The system ofclaim 35 wherein automatically adjusting a value of a signal deliveryparameter includes automatically adjusting at least one of a compliancevalue and a compliance margin in accordance with which the electricalsignal is delivered.
 45. A patient therapy system, comprising: apatient-implantable portion that includes a pulse generator and a signaldelivery device having an electrode positioned to be in electricalcommunication with a target neural population of a patient whenimplanted; an impedance detector operatively coupled to an electricalcircuit that includes the electrode to detect an impedance of theelectrical circuit; and a processor having a computer-readable mediumprogrammed with instructions that when executed: automaticallyidentifies changes in the impedance of the electrical circuit as afunction of time; based at least in part on identifying the changes inimpedance as a function of time, predicts a future value of theimpedance; and based at least in part on the predicted future value ofthe impedance, automatically performs a task.
 46. The system of claim 45wherein the processor is carried by the patient-implantable pulsegenerator.
 47. The system of claim 45 wherein automatically performing atask includes comparing the identified impedance changes with apredetermined schedule of impedance changes.
 48. The system of claim 45wherein automatically performing a task includes automatically issuing anotification.
 49. The system of claim 45 wherein identifying changes inimpedance includes identifying intermittent impedance increases, andwherein predicting a future value of the impedance includes predicting afailure of the signal delivery device.